A reciprocating internal combustion engine is a heat engine that uses one or more reciprocating pistons to convert the energy of a combusting fuel into a rotating motion.
In a typical reciprocating internal combustion engine, the expansion of the high-temperature and high-pressure gases produced by the combustion of fuel inside a cylinder applies force to drive a piston inside the cylinder. This force moves the piston over a distance thereby transforming the chemical energy of the fuel into useful mechanical energy.